Load-Induced Changes in the Diffusion Tensor of Ovine Anulus Fibrosus: A Pilot MRI Study

Purpose: To assess the feasibility of diffusion tensor imaging (DTI) for evaluating changes in anulus fibrosus (AF) microstructure following uniaxial compression. Materials and Methods: Six axially aligned samples of AF were obtained from a merino sheep disc; two each from the anterior, lateral, and posterior regions. The samples were mechanically loaded in axial compression during five cycles at a rate and maximum compressive strain that reflected physiological conditions. DTI was conducted at 7T for each sample before and after mechanical testing. Results: The mechanical response of all samples in unconfined compression was nonlinear. A stiffer response during the first loading cycle, compared to the remaining cycles, was observed. Change in diffusion parameters appeared to be region-dependent. The mean fractional anisotropy increased following mechanical testing. This was smallest in the lateral (2% and 9%) and largest in the anterior and posterior samples (17-25%). The mean average diffusivity remained relatively constant (<2%) after mechanical testing in the lateral and posterior samples, but increased (by 5%) in the anterior samples. The mean angle made by the principal eigenvector with the spine axis in the lateral samples was 738 and remained relatively constant (<2%) following mechanical testing. This angle was smaller in the anterior (558) and posterior (478) regions and increased by 6-168 following mechanical testing. Conclusion: These preliminary results suggest that axial compression reorients the collagen fibers, such that they become more consistently aligned parallel to the plane of the endplates.